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Sep 1, 2006

More data on the Compass Sensor...

Well, I'm away for a day-and-a-half and I return to find weapon systems for NXT! My how this product matures quickly!

After playing around with the compass some more, I realized I wanted a little more detail on how the motors interact (conflict) with the new Compass Sensor. So I did some quick tests:

1. I built a small frame to hold the sensor away from the Brick and one motor.2. I got the Compass Sensor to return a fixed value of 0 (North).3. I then slowly moved the motor towards the sensor and watched the value.

In this figure, the compass still returns a value of 0. The Brick shows 358, but it would change a few times and settle to 0.

In this figure, the motor is slightly closer and the value stayed consistently at 2. Not bad. But at other times, the reading would jump to near 280 and then back to 2. An accurate reading, then an inaccurate one. Maybe some pulses from the motor or compass or both?

In this final image, I held the motor next to the sensor. The value fluctuated between 190 and 220 - a 140 degree difference!! So the lesson is this: Keep that Compass Sensor AWAY from your motors.

I imagine that plugging in more motors and sensors may cause even more trouble. I'll play with the sensor some more and let you know what I find.

15 comments:

Maniac000
said...

This probably isn't so great if the compass sensor can be used in FLL. With the size restrictions on the FLL robot, the compass sensor would probably be close ebough that its reading would be effected.

But the HiTechnic sensors will be sold officialy via Lego Shop @ Home.Wouldn't this be almost the same as a Lego product? But then, there are the restrictions on how many and wich sensors you can use, so you're probably right.

The fact that the RCX doesn't have an "official" compass is probably a good point... but then again, the RCX doesn't have an "official" US sensor either, while the NXT does.

Practically, I suspect that as long as the RCX is allowed, there will be no added sensors to the list. And even after FLL (someday?) switches to just the NXT, cost may be an issue. You want to make sure that all the teams have roughly equal resources, which either means adding a compass (or other) sensor to *everybodies* kit, or not permitting them. Just my guess.

Even though Lego will be selling these third-party sensors, the FLL has always been strictly non-"homebrew" or anything other than that made by lego. Vex is where they do stuff like that. Also, there is no compass sensor block on the NXT-G, plus, like Jim said, they've already relesed the allowed-parts.

As I write on my review of the compass sensors published more then a month ago (read it here), the HiTechnic sensor didn't include filter while mindstorms.com did. I believe the later would have less motor influence then HiTechnic sensor.

Is it my imagination, or does the ultrasound sensor perform better the farther away it is from the motors?

This sounds preposterous, I guess. All I know is that when I attach the sensor "high and out" (i.e, far away from the motors), it's much more accurate in detecting the distance to solid objects. (I assume that this phenomenon has nothing to do with electrical interference, but I'm at a loss to explain it).

As with any compass, external magnetic sources will influence the readings. As a general rule, the compass should be at least 10 – 12 cm (approx 4 –5 inches) away from motors, the NXT and other sources of magnetic interference. Once a model is built the compass can be calibrated for that installation, removing most externally induced errors.

10-12cm should be easy enough to accomplish for most bots. I am absolutely enjoying this new sensor... I've played around with a version of the bot that Filip demonstrated over at bnxt.com... and I can't wait to get the NXT-G block and play around with its features... Steve, has there been any decision about when the Compass Sensor might be available to the public for purchase?

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